It's a weird thing, and pop culture is full of it: straight men fantasizing about lesbians. Why is it?

It's probably about male insecurity. These fantasies have no competing male. Rather they have additional women to implicitly/potentially shower affection on the man.

It's probably got hardly anything to do with lesbianism. It's a fantasy that's not actually interested in female desire. In fact these fantasies presuppose bisexual women who are open to a menage a trois - that is, open to serving male sexuality. If the dream-women excluded and rejected men, the appeal would probably evaporate.

So in that sense, these fantasies are misogynist. If I am right then they are a particular, perverse form of the objectification of the female body and the use of female desire to satisfy male insecurity.